The ¹H-NMR spectrum of [(C₅H₅)₂Fe (CO)₂] exhibits two peaks of equal intensity at room temperature, but four resonances of relative intensities 5:2:2:1 at lower temperature. The hapticities of C₅H₅^- are

Concept:

• Proton nuclear magnetic resonance or 1H NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen nuclei within the molecules of a substance, in order to determine the structure of its molecules.
• Proton NMR spectra are characterized by chemical shifts in the range of +14 to -4 ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

Explanation:

• The compound (C5H5)2Fe(CO)2 contains both \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) and \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ligands.
• The 1H NMR spectrum at room temperature shows two singlets of equal area.
• This is because the five equivalent protons of the \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will show one singlet in 1H NMR spectra.
• While, the \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will also show one singlet in 1H NMR spectra, although the protons are not all equivalent.

• This is because of the “ring Whizzer” mechanism, by which the five ring positions of the monohapto ring of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) interchange via 1,2-metal shift so rapidly at room temperature that the NMR spectrometer can detect only the average signal for the ring.
• At lower temperature, this process is slower, and the different resonances for the protons of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) become apparent at lower temperatures, the peak at 4.5 ppm \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) remains constant, but the other peak at 5.7 ppm for \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) spreads and then splits into four peaks of intensity ratio (5 ∶ 2 ∶ 2 ∶ 1).

​Conclusion:

Hence, the hapticities of C5H5- are η5 and η1